. . "FIGS. 5A to 5E are diagrams showing step by step a manufacturing method of a semiconductor memory device in accordance with the present invention, and FIG. 6 is a plan view of a semiconductor memory device showing one embodiment of the present invention, and the sectional view of the portion taken along the line A--A in FIG. 6 corresponds to FIG. 5E. As shown in FIG. 5A, a silicon oxide film 2 and" . .